1. Field of the Invention
This invention relates to an analog signal processor; and more specifically such a processor, particularly suitable for use in nuclear power plant applications, which converts analog process signals to digital form and employs continuous on-line automatic calibration in order to accurately compensate for gain and bias errors occurring in its input analog circuitry.
2. Description of the Prior Art
In real-time process control systems, a set of analog process signals usually represents corresponding process variables. Each signal emanates from an associated process transducer and is applied as an input to a process controller. Depending upon the particular transducer, the analog process signal can be either an analog voltage of an analog current. A commonly used analog voltage range is 0-10 volts, and a commonly used analog current range is 4-20 milliamperes. Process transducers are usually field-mounted process transmitters, such as pressure sensors, or detectors, such as resistance temperature detectors (RTDs). The process controller continually monitors each analog process signal to determine the actual value of each corresponding process variable. Once the value of each process variable has been ascertained, the controller generates necessary process control signals in order to provide a desired function, i.e. controlling the process in a pre-defined manner or activating an alarm if any analog process signal exceeds a certain set limit value. Most process controllers convert the analog process signals into digital form by a suitable analog-to-digital (A/D) converter for subsequent processing by a digital computer, frequently a microprocessor.
Generally, many controlled electromechanical processes, such as those that frequently occur in a nuclear power plant, change rather slowly. As such, the analog process signals in these processes only contain low frequency components, for example, with a maximum frequency component of less than 10 Hz and frequently less than 1 Hz. However, these signals are often corrupted by low frequency noise, such as induced power line hum and harmonics of the power line frequency. To achieve accurate control, the process controller must first remove this noise from each process signal. This function, as well as the analog-to-digital conversion of the analog process signal, is provided by an analog signal processor that is connected to each incoming analog process signal.
In the past, an active (analog) filter was employed in the analog signal processor in order to remove noise appearing in each incoming analog process signal. Unfortunately, the active circuitry present in these filters introduced undersirable long-term gain and bias (offset) errors resulting from component aging, environmental changes and other factors. This, in turn, limited the stability of these filters and, hence, the accuracy of the filtered analog process signals. Since these filters were positioned ahead of the A/D converter, the converted analog signal often differed, as a result of these errors, by several counts from the true digital equivalent of the analog process signal. Active filters disadvantageously required frequent re-calibration to maintain these differences within an acceptable level.
Circuitry was often included within the analog signal processor to automatically perform this calibration on a regular basis and thereby compensate for the gain and bias errors. Calibration entails disconnecting the analog process signal from its corresponding input terminal and substituting a fixed reference voltage as the analog input signal to the controller and, either manually or automatically, adjusting programmed gain and bias compensation constants in order to produce a desired output signal. Once the reference signal was applied as input to the active filter, no readings of this signal could be taken for a period of time in order to enable the output of the filter to fully stabilize. This time period usually extended to least eight filter time constants for 99.99% accuracy. Inasmuch as the active filters possessed a slow response, often as long as approximately 100 milliseconds for the half step response time, the A/D converter could not sample the reference signal applied to a filter input until at least 800 milliseconds have elapsed. Unfortunately, for certain types of processes, such as monitoring the status of nuclear power plants, analog process signals could not be disconnected from the process controller for more than a short pre-defined time interval, usually no more than 100 milliseconds (1/10 second). Therefore, in these situations, the active filters could not be totally calibrated until required monthly tests are performed during which the process signals could be disconnected from the inputs to these filters for an extended period of time.
As such, the design of analog input circuits for process controllers for nuclear power plant applications frequency relegated the active filter to a position outside an automatic gain and bias calibration loop. In an attempt to minimize gain and bias errors occurring between successive re-calibrations, these filters were implemented using highly stable components. Unfortunately, such components were quite expensive and often failed to maintain these gain and bias errors within an acceptable range.
These analog input circuits possessed other drawbacks as well. First, certain operational failures could not be readily detected. Specifically, an analog multiplexer, located within the analog signal processor but under the control of the microprocessor situated within the process controller, selectively applied either the filtered analog process signal or one of two reference signals as input to the A/D converter. This allowed the process controller to repetitively inject each reference signal into the input of each analog input circuit and determine any conversion errors as being the difference between the true digitized value of the references, previously stored as constants, and the actual output values of the converter. Gain and bias correction algorithms were typically executed to compensate subsequently occurring process signals. In rare instances, the multiplexer used in these circuits failed in one position and was unable to apply the analog process signal in lieu of a reference signal as input to the A/D converter. Unfortunately, either of the reference signals could possess a value lying within the permissible range of the analog process signal. Consequently, when the multiplexer failed in this manner, the process controller would detect what it believed to be a steady normal value of the process signal, when, in fact, that reference signal was being applied instead. Hence, the controller was totally unaware of this failure and drove the controlled process toward an abnormal state.
Second, these analog input circuits could not be used with a wide variety of different RTDs without incurring significant expense. Specifically, an RTD, as noted above, is frequently used to measure a process temperature and, for that reason, is connected as an input transducer to the analog input circuit. Different RTDs are used to measure different temperature spans. These RTDs may all be one type, i.e. having the same temperature coefficient of resistivity, or a variety of types. As many as 25 different temperature spans may be measured in a typical control system for a nuclear power plant. With a constant current, generally 1 milliampere, flowing through every RTD in a system, each of these RTDs will produce a output voltage range dependent upon its type and the temperature span it is measuring. Hence, the voltage span produced by one RTD will not generally be the same as that produced by another. Consequently, a unique RTD input circuit must be used within the analog input circuit for each different temperature span and each different type of RTD, in order to appropriately scale the output of each amplified RTD signal to a uniform range, such as 0-5 volts. Unfortunately, this necessitates that a large variety of RTD input circuits, each with different components, be made available for use with the analog signal processor whenever RTDs are to be employed as input transducers. Circuits of this type generally utilize odd value precision resistors and unfortunately tend to be quite expensive.
Therefore, a need exists in the art for an analog signal processor, particularly one suitable for use in nuclear plant applications, which converts analog process signals to digital form and employs continuous on-line automatic calibration in order to accurately compensate for gain and bias errors. In addition, this processor should filter noise from each process signal and also fully detect operational failures in its input multiplexers and be capable of accepting a multitude of different RTDs as input without requiring a large number of specialized and expensive RTD input circuits.